1. Field of the Invention
The present invention relates to laboratory testing apparatuses and particularly to apparatuses and methods for determining the effectiveness and reaction time of an oil well bore hole filter cake breaker fluid.
2. Description of Relevant Art
Drilling fluid or drilling xe2x80x9cmudxe2x80x9d is used in drilling oil and gas wells to serve several purposes, including lubricating the drill bit, removing cuttings form the wellbore, and providing stability to the well. To accomplish its purposes, drilling fluid typically possesses several characteristics. One such characteristic is the ability if desired to form a coating or xe2x80x9cfilter cakexe2x80x9d on the wall of the wellbore or borehole. The filter cake serves to stabilize the borehole and prevent loss of the liquid portion of the drilling mud through the walls of the borehole into the adjoining formations. This loss of liquid from the drilling mud, commonly referred to as fluid loss, is a function of many variables such as the composition of the drilling mud, the types of formations encountered in the drilling process, temperatures and pressure in the borehole, etc.
The ability to test drilling muds under simulated borehole conditions to determine characteristics such as fluid loss is desirable and apparatuses for such testing have been demonstrated. See for example, U.S. Pat. No. 4,748,849, issued Jun. 7, 1988 to Jamison et al. Commercial equipment is also available. Fann Instruments Company, a Halliburton Company, provides a Model 90, Dynamic Filtration Tester, that is particularly effective for dynamically measuring fluid loss characteristics.
Although a filter cake may be desirable during drilling a wellbore, removal of the cake is frequently desirable after the drilling is complete as the filter cake may interfere with production of oil and gas from the formation into the well. Thus, removal of filter cake from drilling fluids used to drill through producing zones of the formation is usually particularly desirable.
Many highly permeable, softer sandstone formations are completed with horizontal wellbores. These are generally open hole completions with screens, or screens-plus-gravel packs. The filtercake from the drilling fluid or xe2x80x9cdrill-inxe2x80x9d fluid (as drilling fluids used for drilling through production zones may also be called) is left in place until after completion operations are finished. When the completion operations are done, a cleanup solution is pumped into the well to remove the filtercake from the wellbore face. These cleanup solutions can be, for example, acid, chelating agents, oxidizers, enzyme treatments, or combinations thereof A common disadvantage of any of these treatments is that the cleanup solutions tend to be highly reactive and may remove the filtercake at the point of circulation before the treatment can be placed over the entire open hole interval. This problem is especially common with clean-up solutions used to address the bridging particles used in payzone or xe2x80x9cdrill-inxe2x80x9d fluids.
There is a need for cleanup solutions that have a delayed effect on filtercake integrity. Such delay allows the cleanup solution to be circulated across the interval before xe2x80x9cleakoff,xe2x80x9d or before flow from a portion of the formation with filtercake removed occurs. Ideally, once the cleanup solution or filter cake breaker solution is in place in the wellbore, the cleanup solution begins reacting with the filtercake to remove it. If the delay could be made sufficiently long enough, the breaker solution could be placed with the gravel pack. Break time for this type application would need to be in the range of about four hours to as long as about twenty-four hours depending on the desire or need to have fluid loss control during subsequent pipe trips.
Conducting breaker tests for this type of delay could be extremely time consuming and prone to variations from test to test if conducted manually. Repeatability of the filtercake deposition becomes even more important when conducting sensitivity testing of the various components of a proposed cleanup solution. A need exists for automating filter cake construction and beaker solution testing.
For purposes of the present invention, known equipment for dynamically measuring fluid loss characteristics can be adapted to test filter cake breaker fluid. U.S. Pat. No. 4,748,849, for example, discloses an apparatus for dynamically testing fluid loss characteristics of a test fluid such as drilling mud. This apparatus comprises a filter medium cell with a generally vertically disposed, cylindrical wall section. The wall section has an inner surface and an outer surface. At least part and preferably all of the wall section is formed of a filter medium. A first chamber for a test fluid is in open communication with the inner surface of the cylindrical wall section of the cell. A second chamber is in open communication with the outer surface of the cylindrical wall section of the cell. A generally vertically disposed rotatable shaft is at least partially received in the filter medium cell and a rotator or other means is provided or included to rotate the shaft. The apparatus further includes a means to apply a differential pressure across at least a portion of the cylindrical wall section between the first chamber and the second chamber through the filter medium portion of the wall section. Means is also provided for measuring the amount of test fluid which passes from the first chamber to the second chamber through at least the filter medium portion of the wall section.
To adapt such an apparatus as disclosed in U.S. Pat. No. 4,748,849 for use in the method of the present invention, the apparatus is preferably fitted with a manual or automatic valve in the filter line to reduce the possibility of filter cake damage if and when the apparatus is opened. Also, preferably, a guide is added to prevent the shear shaft from damaging the filter cake if and when the shaft is removed after forming the filter cake.
In the method of the present invention, a dynamic filter cake is formed in such adapted apparatus of U.S. Pat. No. 4,748,849 or similar apparatus capable of forming a filter cake with drilling mud at a desired temperature and differential pressure, such as a temperature and pressure expected to be encountered in a subterranean wellbore. Filter cake breaker fluid is then substituted in the apparatus in place of the drilling mud. Such substitution may typically be effected by opening the apparatus and removing the drilling mud, by for example pouring the drilling mud out and replacing it with breaker fluid, or by for another example allowing the drilling mud to drain out into an auxiliary fluid container and adding the breaker fluid from another auxiliary fluid container, preferably without having to depressurize or cool the apparatus system. Collection of filtrate is begun, resumed or continued (filtrate likely having been collected during formation of the filter cake), and the quantity may be monitored over time. An increase in the filtration collection rate indicates filter cake removal. The greater the increase or rate of change, and the faster the flow of fluid, the greater the degree of filter cake removed.